Current limiting fuse including radially disposed fuse elements

ABSTRACT

A current limiting fuse having one or more fuse elements disposed for more efficient transfer of heat so that greater electrical current may be continuously conducted through the fuse for a given cross-section of fusible material. Two electrodes or terminals are provided, one of which is cup-shaped or which is tubularly shaped and closed by electrically conducting material at one end. The second electrode projects into the recess created by the cup-shaped electrode and is spaced from it. An electrically insulating support-cover or spacer is provided at the open end of the cup-shaped electrode. This spacer has a central hole or opening to accommodate the projecting electrode or terminal. An array or assembly of fuse elements is disposed upon the enclosed portion of the protruding electrode. Each fuse section is joined at its other end to the inner perimeter of the cup-shaped electrode. The fusible elements may be arranged as spokes on a wheel between a central hub and an outer ring, the central hub being mounted on the inwardly protruding electrode and the outer ring being disposed to make contact with the outer or cup-shaped electrode. The enclosure formed by the cup-shaped electrode, the insulating spacer and the protruding electrode is filled with a granular pulverulent such as quartz sand for electrical arc quenching.

United States Patent 1 Blewitt et al.

[ 1 May 8, 1973 [54] CURRENT LIMITING FUSE INCLUDING RADIALLY DISPOSED FUSE ELEMENTS 75 Inventors: Donald 1). Blewitt, Pittsburgh; Woodrow G. Shaw, Murrysville,

both of Pa.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

221 Filed: Feb. 25, 1972 211 Appl.No.: 229,363

[52] U.S. Cl ..337/166, 337/293 [51] Int. Cl. ..I*I0lh 85/04 [581 Field of Search. ..337/l59, 166, 290,

[56] References Cited UNITED STATES PATENTS vex IZA] A? [57] ABSTRACT A current limiting fuse having one or more fuse elements disposed for more efficient transfer of heat so that greater electrical current may be continuously conducted through the fuse for a given cross-section of fusible material. Two electrodes or terminals are provided, one of which is cup-shaped or which is tubularly shaped and closed by electrically conducting material at one end. The second electrode projects into the recess created by the cup-shaped electrode and is spaced from it. An electrically insulating support-cover or spacer is provided at the open end of the cup-shaped electrode. This spacer has a central hole or opening to accommodate the projecting electrode or terminal. An array or assembly of fuse elements is disposed upon the enclosed portion of the protruding electrode. Each fuse section is joined at its other end to the inner perimeter of the cup-shaped electrode. The fusible elements may be arranged as spokes on a wheel between a central hub and an outer ring, the central hub being mounted on the inwardly protruding electrode and the outer ring being disposed to make contact with the outer or cup-shaped electrode. The enclosure formed by the cup-shaped electrode, the insulating spacer and the protruding electrode is filled with a granular pulverulent such as quartz sand for electrical arc quenching.

10 Claims, 7 Drawing Figures H3A I 1Q 18 PATENTEDH-AY SHEET 3 [IF 3 CURRENT LIMITING FUSE INCLUDING RADIALLY DISPOSED FUSE ELEMENTS CROSS-REFERENCE TO RELATED INVENTIONS Certain inventions related to those disclosed in the present application are disclosed and claimed in copending application Ser. No. 229,362 filed concurrently by D. D. Blewitt and assigned to the same assignee as the present application.

BACKGROUND OF THE INVENTION This invention relates generally to current limiting fuses and it has particular relationship to radially disposed fuse elements.

Current limiting fuses may comprise fuse elements including strips or ribbons of silver-based alloy fusible material having areas of reduced cross section. Electrical current flowing through the circuit to be protected by the fuse must of necessity flow through the regions of reduced cross section. These are regions of high current density and consequently are regions where heat may be generated and the temperature increased because the surface area of the fusible material in such a region is insufficient to dissipate the heat generated by the flow of current through it. Should the electrical current reach a sufficiently high value, the regions of reduced cross section may melt or fuse. Such an occurrence is a proper protective function of the fuse. How ever, if the heat generated in the regions of reduced cross section of fused material is quickly removed from the regions of reduced cross section the temperature of the region willbe lowered and the current at which the fusible material may melt or blow is increased. This means that more current may be accommodated through the regions of reduced cross section without the fuse melting or said another way, it provides for a fuse having a higher rated current capacity eventhough the cross sectional area of the fusible elements has not been changed.

Heat is best removed by conduction from the area or areas of reduced cross section through the fusible material to an adjacent electrode which acts as a heat sink. Some but very little heat may be removed from the fusible material through adjacent granular arc quenching material to an insulating tube which may be axially disposed between the spaced terminals or electrodes. Elements of fusible material may be mounted between the spaced terminals in an axially disposed parallel electrical and geometric configuration and surrounded by the previously mentioned granular material. Since the fuse elements are oriented in a parallel, geometric configuration they are relatively crowded within a confined space equally along their entire length and heat conduction for each element through the granular medium is uniformly poor because of the presence of similar proximate heat producing fusible elements all of which in combination may provide relatively large amounts of heat to a relatively small volume of adjacent pulverulent material for further conduction to the outer casing which itself may be a poor conductor of heat. Such being the case, the primary means for removing heat is by conduction through the fuse material to the associated end terminals or electrodes. It would be desirable to provide a current limiting fuse structure having improved heat transfer characteristics and which would be more compact than known fuse structures of the same general type. Radially oriented fuse elements have been proposed in U.S. Pat. No. 2,000,016 issued May 7, 1935 to William H. Frank et al. and U.S. Pat. No. 2,106,109 issued Jan. 18, 1938 to Pierce Prendergast. In both these patents, however, electrical current is not meant to be conducted concurrently through all of the radially oriented fuse members. On the contrary current is meant to be conducted through one fuse member at a time. When that fuse member blows or melts due to overcurrent, the barrel or casing of the fuse may be rotated to replace the spent fuse section by another radially oriented fuse element thus rendering the overall fuse operable once again.

SUMMARY OF THE INVENTION The disclosed invention relates to a current limiting fuse having a central electrode or terminal upon which is mounted a spoked, wheel-like fuse element assembly having radially oriented, parallel, electrically conducting paths which simultaneously conduct electrical current from the central electrode through the hub of the fuse section to each of the radially disposed fuse members and then to the rim of the fuse element assembly. The electrical current is then transferred to a cylindrical casing or outer electrode or terminal for continued flow in the protected circuit. The spoked or radially oriented fuse element assembly is mounted within the associated tubular electrode. The generally tubular electrode or terminal is closed over at one end by electrically conducting material which may be formed integrally with the tubular portion so as to form an overall cup-shaped electrode. A spacer or electrically insulating and enclosing support member may be provided over the other end of the cylindrical or tubular electrode. It may accommodate the central electrode through a small opening. The enclosed volume may then be filled with a pulverulent arc quenching material which may also act as a heat conducting medium or means. Since the outer casing is both electrically conducting and heat conducting material and has a relatively large surface area, heat generated within each of the spoked fuse elements may rush or flow more easily to the effective heat sink or dissipation means provided by the cylindrical casing. In addition as the heat flows outwardly in a radial direction from the central hub it may be carried or conducted through the adjacent pulverulent material to those parts of the outer casing which are in contact with the granular material but which do not make direct contract with the spoked fuse section. As the radial elements extend outwardly from the central hub, the circumferential distance between them gradually increases and consequently the potential for removing heat from the adjacent fuse material is improved because there is more granular material to accommodate the flow of heat than there is near the hubof the wheel or spoke-like fuse element assembly where the fuse elements are closer together. In certain higher voltage applications, the central electrode may be spaced close to the bottom or conducting end of the cup-shaped electrode, so that are over or flashover may occur between one electrode and the other after the fuse has been spent. To prevent this an electrically insulating member which may also be a good conductor of heat may be provided or disposed between the bottom or conducting end of the cup-shaped electrode and the protruding central electrode to decrease or substantially prevent the possibility of an arc over while maintaining a heat conducting path between the granular material adjacent to the radial fuse sections and the bottom of the cup-shaped electrode. Each radial fuse element may be maintained at a higher temperature in those areas of reduced cross-section closer to the central hub than those areas of reduced cross-section further removed radially from the central hub because of the heat conducting characteristics of the cylindrical outer wall and the larger spacing between radial fuse sections at a further distance from the central hub. Consequently, one of the fuse elements will most likely fuse or break down in the area or region closer to the central hub before it will break down or fuse in a region further removed from the central hub. In quick sequence, once one fuse element has melted all the others will normally be further overloaded and electrically fuse. If desired, the reduced areas in the fusible element may be graded, or varied, along its length to provide for a uniform temperature distribution and hence insure melting at a point midway between the control hub and outer wall.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention, reference may be had to the preferred embodiments exemplary of the invention shown in the accompanying drawings in which:

FIG. 1 shows a side section of a current limiting fuse element with a radially oriented fuse element assembly embodying the principal features of the invention;

FIG. 2 shows a side section of a current limiting fuse element with a radially oriented fuse element assembly;

FIG. 3 shows a side section of a current limiting fuse structure having a plurality of radial elements disposed about a central hub;

FIG. 4 shows a current limiting fuse structure similar to the one shown in FIG. 3 with an additional insulating member;

FIG. 5 shows a fuse element assembly suitable for use in one embodiment of the invention;

FIG. 6 shows a side view of the fuse element assembly shown in FIG. 5; and

FIG. 7 shows a current limiting fuse structure including the fuse element assembly shown in FIGS. 5 and 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings and FIG. 1 in particular, a fuse 10 is shown. Fuse 10 comprises a generally cup-shaped electrode 12 having parallel sides 12A and 128 which together form a generally tubular portion of the electrode or terminal 12 and a bottom or end section 12C. Fuse structure 10 may be tubular, hollow or cylindrical. Electrode 12 may be maintained at :1 voltage V2 and be adapted to carry an electrical current such as I1. Cup-shape electrode 12 may be formed from an electricallyconducting and heat conducting material, such as aluminum, brass, copper or steel based alloys. A second central electrode is disposed to project within the recess formed by the cup-shaped electrode 12. Electrode 14 projects through an electrically insulating disc or supporting spacing structure 16 which has a central opening 13 through which the projecting portion of electrode 14 extends. The end 15 of electrode 14 is axially spaced from the bottom or end portion 12C of the cup-shaped electrode 12. Electrode 14 may be maintained at a voltage V2 such as when the fuse element 18 is normally conducting and has not blown or when fuse element 18 has blown or fused. Electrode 14 may also accommodate or carry electrical current I1.

Fuse element 18 may be mounted to extend in a generally radial direction between electrode 14 in the region of end 15 and the cup-shaped electrode 12 in the region of side or cylindrical portion 12A. The fuse element 18 may be silver-based alloy or any electrically conducting material which is adapted to fuse or blow under predetermined overload current conditions. The fusible material from which the fuse element 18 is formed may be a strip ribbon or tapered in configuration and is joined to electrodes 12 and 14 by a suitable fastening means 19 such as solder or a brazing material. The amount of electrical current I1 flowing through fusing section 18 will determine the amount of heat which will be generated within fuse element 18. The heat may be removed from fusible material 18 through a heat conducting path H1 to the outer shell of the cupshaped outer electrode 12 which may for example be a brass casting whereupon it will spread such as is indicated by paths I-IlA, H18 and HIC throughout the volume of the heat conducting electrode 12. Heat may also radiate or conduct through the enclosing medium M as shown by heat removal path H2. Path H2 may be a convection or conduction path or a radiation path depending upon the quantity and quality of the enclosing medium M which may be a pulverulent arc quenching material, such as silica sand. Heat path H2 may be broken up into three component paths H2A, H213 and H2C which provide heat removal to the regions 12A, 12C and 12B, respectively. The outer surface of the cup-shaped electrode 12 may then radiate heat to the atmosphere or adjacent heat sinks where provided as indicated by arrows H3A, H38 and H3C from the regions 12A, 12C and 128, respectively. It will be noted that the heat removed in each of these regions 12A, 12C and 128 has a component of heat that may be conducted through the fuse element 18 such as indicated by arrows HlCl and HICZ and a component of heat which is convected, conducted or radiated heat through the medium M, such as indicated by HZA, H2B and H2C, respectively.

Referring now to FIG. 2, a fuse structure 20 similar to fuse structure 10 shown in FIG. 1 is shown. The central electrode 14 projects axially farther into the recess formed by the cup-shaped electrode 12 so that the end 15 of the electrode 14' is relatively closer now to the inner surface 17 of bottom or end portion 12C of the outer generally tubular electrode 12. The importance of this construction arrangement lies in the improvement of the heat conducting paths through the arc quenching medium M as indicated by heat removal arrow H4 which has components H4A, H48 and H4C.

As a consequence, the amount of heat HSA, HSB and HSC which may be dissipated or removed from the surface of the generally tubular or cylindrical outer electrode 12 is increased or relatively greater. Such being the case, then fuse element 22 which is formed from fusible material which may be approximately the same size, shape and having the same electrical characteristics as fuse element 18 as shown in FIG. 1 but which may continuously conduct more current I2 than the current I1 as shown in FIG. 1 because the heat generated within fuse element 22 is more rapidly and efficiently removed through the medium M due to the closer proximity of the fuse element 22 to the end wall portion 17 of the electrode 12 for example.

Referring now to FIG. 3, a current limiting structure 30 similar to the fuse structures and shown in FIGS. 1 and 2, respectively, is depicted. Fuse 30 has an improved heat removing fuse element assembly 32. Fuse element assembly 32 has a central hub 36 and an outer rim 38. Central hub 36 fits on and is electrically connected to or in contact with central electrode or electrical conductor 14 with a suitable securing means, such as solder 40, disposed between the hub 36 and electrode 14 to improve the electrical conducting characteristics at the interface between hub 36 and electrode 14. In addition, the ring-shaped or discshaped rim 38 is secured by a similar means such as solder 40 to the outer cylindrical wall of cup-shaped electrode 12. Many electrically conducting paths may exist between electrode 14 and outer cup-shaped electrode 12. Two such paths are indicated as fuse elements 34T and 34B. Fuse element 34T may conduct or carry a certain portion or amount of current l3T and fuse element 34B may carry or conduct a similar amount of electrical current I3B. Each fuse element 34T and 34B may include at least one region or portion of reduced cross-sectional area such as indicated at 42. Region 42 represents a relatively narrow electrically conducting portion through which electrical current such as l3T or [38 must flow. Consequently, heat buildup occurs causing the temperature near this region to rise to a greater degree. If the heat is not substantially removed, the fuse elements 348 and 34T will blow or melt in the regions 42. As shown by heat conducting paths H6T and l-l6B, as an example, heat may be removed by conduction to the outer surface as was previously described with respect to FIGS. 1 and 2. In addition, heat may be removed by conduction through medium Ml, which may be a granular or pulverulent arc quenching material, such as quartz sand, indicated by arrows or heat transfer paths H7T and H7B which respectively may form components H7TA and H7TB as well as H7BA and l-I7BB. The pulverulent material may act as a heat conducting medium for heat build up or accumulate within fuse elements 34T and 34B. Since fuse elements 34T and 34B are only exemplary sections of fusible material connected between hub 36 and rim 38 it can be envisioned that other similar fuse elements may be circumferentially spaced at equal or unequal intervals or angles around the central hub 36. The spac ing between adjacent fuse elements such as 34T or 348 is relatively larger near rim 38 and consequently the heat removal in that region is better than near hub 36 and the temperature of a fuse link such as 34T or 34B is lower near the heat such as indicated by H7T or H7B may be removed more efficiently through the adjacent quartz sand. The outer shell 12 may then radiate or conduct heat outwardly such as indicated by arrows H9 and H8. The more heat which is conducted from the outer surface of cylindrical electrode 12, the larger the amount of current l3 which may be continuously carried or accommodated by the fuse structure 30.

FIG. 4 shows a current limiting fuse structure 50 similar to fuse structure 30 shown in FIG. 3 except that a disc-like dielectric medium such as high alumina ceramic is placed or disposed between the inner perimeter 17 of the bottom section 12C of outer or cup-shaped electrode 12 and the end 15 of centra electrode 14. Consequently, the electrical insulation between voltages V1 and V2 when fuse element assembly 32 has disintegrated or fused is improved. Heat such as indicated for example by heat path H7TB may be conducted through the ceramic or insulating material in a similar fashion to theway heat is conducted through path H7TB as shown in FIG. 3 even though there is now improved electrical insulation.

Referring now to FIG. 5, a unique machine stamped, spoked, wheel like fuse assembly 60 comprising a central hub or ring an an outer rim or ring 62 is shown. Central hub 70 has an inner circular perimeter 74 suitable for mounting on an electrode such as central electrode 14' shown in FIG. 7. The outer perimeter or circumference 64 of rim 62 is suitable for convenient fitting to the inside surface of an outer generally circular or cylindrical tubular cup-like electrode such as 12 shown in FIG. 7. It will be noted that hub 70 has an outer perimeter or circumference 72 and rim 62 has an inner perimeter 68. In this particular embodiment there are eight equally circumferentially spaced fuse elements 76A, 76B, 76C, 76D, 76E, 76T, 76F and 760, disposed in a radial fashion between the central hub 70 and the outer rim 62. Each fuse element may have a plurality of or at least one area of reduced cross-sectional area such as indicated at 78A, 78B, 78C and 78D for fuse link 76T. These areas of reduced cross-section are locations where heat may build up or be generated when an electrical current such as current l4T flows through a radially disposed fuse element such as 76T. The heat may be conducted away from the areas of reduced cross section 78A, 78B, 78C and 78D in a manner indicated by heat conducting paths HlOT. It will be noted that the regions between radially disposed fuse elements increases in a direction away from the central hub 70. Consequently, heat may be more easily removed through the pulverulent conducting medium such as M1 as shown by heat removal path 78E in the region of reduced cross section 78A. Heat may also be removed from the region of reduced cross section 78D as shown by heat removing path 78F although not as efficiently because of crowding.

Referring now to FIG. 6 a side elevation of the spoked wheel-like fuse structure 60 shown in FIG. 5 is depicted. It will be noted that two radially disposed conducting fuse links 76T and 76B are shown. These elements are bowed or arcuate to allow for thermal expansion along the spoke line fuse section so that point 7ST in fuse element 76T for example may move depending upon the amount of heat generated within fuse element 76T. This likewise applies for point 758 on fuse element 7613.

Referring now to FIG. 7 a fuse structure 80 including the spokelike fuse element assembly 60 shown in FIGS. 5 and 6 is depicted. Fuse structure 60 has an outer cup-like electrode 12 which is generally circular or cylindrical and tubular and has two shoulders or ridges 12A, 12B and a shoulder portion 17' (which is also the bottom or end portion of the cup-like electrode 12'). The fuse structure 60 also includes a disc-shaped insulator 16' which may act as both a supporting and enclosing structure. The insulator l6'.may be formed from glass melamine or some similar electrical insulating material and includes a hole 13' through which an electrode 14' projects into the recessed opening formed by cup-shaped electrode 12. Electrode 14' is mounted flush against insulator 16 at a lip or shoulder 14A. Insulator 16' also rests on lip or shoulder 12A which as has been described is provided at the inner periphery of electrode 12. The generally radially fuse disposed assembly 60 is disposed on lip 12B of cupshaped electrode 12 and the hub 70 is disposed against lip or edge 14B of electrode 14. Suitable securing means such as solder 19 is used to join the rim 62 and the hub 70 to the outer electrode 12 and inner insulating electrode 14', respectively. The end 15' of central electrode 14' is mounted flush against an electrically insulating member 52'. The other side of insulating member 52 is mounted flush against the bottom or end portion or base 17 of the outer cup-shaped electrode 12. A silicon grease suitable for increased heat conduction may be placed along the interface 17' between electrode 12' and insulating member 52. In a similar manner a similar grease may be placed between end 15' of electrode 14 and insulator 52'. Electrical current I4 may flow through electrode 14' and through the radially disposed fuse elements, such as 76T and 768 to outer electrode 12. Heat built up within fuse link 76T for example may be removed along heat conducting path HT which may be divided into HlOTC, HIOTB and HlOTA when the heat reaches the outer electrode 12'. In addition, heat may be conducted away from fusible material 76' through pulverulent arc quenching material Ml which may be quartz sand as shown by path Hl 1T which may be divided into component paths HllTA and HllTB. Consequently, heat conducting paths to the atmosphere, ehvironment or other heat conducting means are shown by H13 and H12, which heat conducting paths remove heat which builds up in or is sinked or conducted in some matter from electrode 12'. Spacer or insulator 16 may be retained in position by a retaining ring 82. In addition, tapped or drilled holes 84 may be placed in a portion of electrode 12' for fastening to a support means, heat sink means or electrically conducting means. Although not shown, electrode 14 may be of circular metallic stock or hex shaped stock or any other suitable stock, and may have a tapped hole near end 90 or may have threads cut on end 90 for mounting onto an adjacent external electrical conductor.

In summary, a currentlimiting fuse structure as dis closed includes a fuse element assembly which in general is radially disposed between a central electrode or terminal and an outer generally tubular electrode or terminal which in certain embodiments includes an end portion or member which may be formed from a material which is both electrically and thermally conducting. The end portion of the outer electrode is disposed generally structurally parallel to the associated fuse element assembly and is spaced therefrom by a distance which is relatively less than the radial distance or spacing between the central electrode and the generally tubular portion of the outer electrode to reduce the lengths of any heat transfer paths between the fuse elements of the fuse element assembly and the end portion or heat radiating or transfer surface of the outer electrode through the associated arc quenching material or other electrically insulating member where provided in a particular embodiment.

It is to be understood that the fuse element assembly 60 may be silver or silver alloy based material or tin based material or any other material suitable for providing a current limiting type fuse operation which may melt and arc or flashover upon the application of overload current to a fuse in which it is placed. It is also to be understood that the number of radial elements such as 763 and 76T as shown in FIG. 5 may be increased or decreased to any practical number suitable for conducting electrical current through the fuse element assembly and disposing of built up or generated heat by means of convection or radiation in a heat conducting manner. It is also to be understood that the outer shell or electrode 12 may be formed by any manufacturing process and may be of any suitable electrically and heat conducting material such as brass. In

addition, electrode 14' may be a suitable machined or cast member formed from copper, aluminum steel or brass, for example. Ceramic material 52 and material 52' may be substantially heat conducting but not substantially electrically conducting.

The apparatus embodying the teachings of this invention has several advantages, the first of which is the fact that the outer containing vessel or electrode may be electrically conducting as wellas being heat conducting. Consequently, an electrode such as 12 may conduct electricity, may conduct heat away from fuse links and may contain an insulating or pulverulent arc quenching material such as M or Ml simultaneously. Another advantage lies in the fact that the fuse elements are radially disposedLarge volumes of heat dissipating material are available at the further extension of each radial fuse element so that heat may be conducted from the fuse elements through the pulverulent material and to adjacent portions of the electrically conducting outer shell very easily. Heat may be removed by the surrounding environment and it may be removed to an outer casing in a radial manner and therefore more easily dissipate the heat to the surrounding environment. Consequently, the total current which a fuse embodying the spoke or wheel-like fuse element assembly shown in embodiment in FIG. 5 may carry more electrical current without fusing, disintegrating or breaking down compared with a fuse element assembly having an equivalent total cross section but arranged to extend axially within a cartridge type casing or container and arranged in a parallel or sideby-side relationship where the heat built up near-the disposed parallel conductors in the pulverulent material acts as a dam or stop for radial movement of heat because the arc quenching material or quartz sand has been saturated with heat to bring it to almost the same temperature as the fuse elements which it surrounds. Another advantage of the preferred embodiments of the invention lies in the ease of assembly. The circular fuse element for example may be stamped in one operation and then soldered into a circular precast cuplike electrode. The central electrode may be placed and supported by the disc-like end-covering or enclosing insulator 16' in semi-automatic fashion and in fact may be keyed to the outer shelf 12 so that it will not rotate once assembled. Another advantage lies in the large heat transferring or radiating surface area provided by the outer periphery of the cup-like electrodes 12 or 12'. The later surface presents to the atmosphere a total surface for the dissipation of heat significantly larger than is the case with a similar fuse employing spaced generally equally size electrodes or terminals axially separated by a cylindrical axially disposed electrically insulating casing or housing and having disposed there between parallel fuse sections. The disclosed invention has the additional advantage of being structurally stronger and more secure because the side members such as 12A and 12B comprise metallic members rather than an electrically insulating member such as glass melamine. In addition a portion of the cupshaped electrode such as 12C may be cast integrally with generally cylindrical, hollow tubular side sections 12A and 12B of electrode 12 as shown so as to make the structural strength of the resultant cup-shaped electrode greater than that of ferrules attached to an insulating cylinder. Also, the central opening or recess formed by the cup-shaped member 12 is closed off by the insulating member such as 16' and the resultant opening may be filled with silica or quartz sand through a fill hole 88 in the outer electrode 14 in a uniform manner to reduce the splash area and thus insure a more uniform, or denser, sand fill than possible with cylindrical fuses.

We claim as our invention:

1. A current limiting electrical fuse structure com-' prising in combination first and second electrodes, electrically insulating means, and a fusible element assembly, said first electrode having a recessed opening, said insulating means being interposed between said first and second spaced electrodes to cover said opening, a portion of said second electrode projecting into said recessed opening through said insulating means, said fuse element assembly being interconnected electrically between said first electrode and the said portion of said second electrode which projects into said recessed opening, heat generated in said fuse element assembly being conducted through a portion of said fuse element assembly to said first electrode for dissipation.

2. The combination as claimed in claim 1 wherein said first electrode comprises an electrically and heat conducting tubular means, the end of said tubular means opposite the end where said second electrode projects into said opening being generally occupied by an electrically and heat conducting means.

3. The combination as claimed in claim 2 wherein a portion of said fuse element is disposed proximate to the said electrically and heat conducting means occupying the latter mentioned said end of said tubular means so that heat may he removed from said fuse element through the region between the said fusible element and the latter mentioned heat conducting means to the latter mentioned heat conducting means for dissipation.

4. The combination as claimed in claim 3 wherein said fuse element comprises a plurality of elements of fusible material, a hub and a rim, said elements of fusible material being disposed to extend radially from said hub to said rim, said hub being disposed on said second electrode and said rim being disposed adjacent to said tubular means.

5. The combination as claimed in claim 4 wherein a pulverulent arc quenching material is disposed proximate to said elements of fusible material, said elements of fusible material each having one or more regions of reduced cross section.

6. The combination as claimed in claim 5 wherein said electrically and heat conducting means occupying the said end of said tubular means has disposed adjacent to it in a portion of said region between the fusible material and electrically and heat conducting means, an electrically insulating, heat conducting means, said latter aiding in moving heat from portions of said fusible material to said electrically conducting and heat conducting means.

7. The combination as claimed in claim 6 wherein said electrically insulating, heat conducting means comprises refractory alumina ceramic material. ,1

8. The combination as claimed in claim 7 wherein said tubular means and said electrically conducting heat conducting means occupying said end of said electrode form an integral unit.

9. The combination as claimed in claim 8 wherein said integral unit comprises cast brass material, wherein said tubular portion comprises a cylindrical shell having an inner perimeter, said second electrode being spaced generally equally from said inner perimeter of said cylindrical shell, said insulating means having a central opening and an outer periphery, said outer perimeter being disposed to allow said second electrode to pass through it and to simultaneously support said second electrode, said outer periphery of said insulating means being complementary in size and shape generally to said inner periphery of said cylindrical shell, said insulating means being in turn supported by said cylindrical shell.

10. The combination as claimed in claim 9 wherein said last mentioned insulating means comprises glass melamine, and fusible elements, said rim and said hub comprising an integrated stamped silver based material adapted to be mounted perpendicularly and radially on said second electrode and to be easily soldered to said second electrode and said cylindrical shell, a retaining ring, said last mentioned insulating means being keyed to said cylindrical shell to prevent substantial rotary movement after assembly and retained in a generally fixed longitudinal position by said retaining ring disposed on said inner perimeter of said cylindrical shell. 

1. A current limiting electrical fuse structure comprising in combination first and second electrodes, electrically insulating means, and a fusible element assembly, said first electrode having a recessed opening, said insulating means being interposed between said first and second spaced electrodes to cover said opening, a portion of said second electrode projecting into said recessed opening through said insulating means, said fuse element assembly being interconnected electrically between said first electrode and the said portion of said second electrode which projects into said recessed opening, heat generated in said fuse element assembly being conducted through a portion of said fuse element assembly to said first electrode for dissipation.
 2. The combination as claimed in claim 1 wherein said first electrode comprises an electrically and heat conducting tubular means, the end of said tubular means opposite the end where said second electrode projects into said opening being generally occupied by an electrically and heat conducting means.
 3. The combination as claimed in claim 2 wherein a portion of said fuse eLement is disposed proximate to the said electrically and heat conducting means occupying the latter mentioned said end of said tubular means so that heat may be removed from said fuse element through the region between the said fusible element and the latter mentioned heat conducting means to the latter mentioned heat conducting means for dissipation.
 4. The combination as claimed in claim 3 wherein said fuse element comprises a plurality of elements of fusible material, a hub and a rim, said elements of fusible material being disposed to extend radially from said hub to said rim, said hub being disposed on said second electrode and said rim being disposed adjacent to said tubular means.
 5. The combination as claimed in claim 4 wherein a pulverulent arc quenching material is disposed proximate to said elements of fusible material, said elements of fusible material each having one or more regions of reduced cross section.
 6. The combination as claimed in claim 5 wherein said electrically and heat conducting means occupying the said end of said tubular means has disposed adjacent to it in a portion of said region between the fusible material and electrically and heat conducting means, an electrically insulating, heat conducting means, said latter aiding in moving heat from portions of said fusible material to said electrically conducting and heat conducting means.
 7. The combination as claimed in claim 6 wherein said electrically insulating, heat conducting means comprises refractory alumina ceramic material.
 8. The combination as claimed in claim 7 wherein said tubular means and said electrically conducting heat conducting means occupying said end of said electrode form an integral unit.
 9. The combination as claimed in claim 8 wherein said integral unit comprises cast brass material, wherein said tubular portion comprises a cylindrical shell having an inner perimeter, said second electrode being spaced generally equally from said inner perimeter of said cylindrical shell, said insulating means having a central opening and an outer periphery, said outer perimeter being disposed to allow said second electrode to pass through it and to simultaneously support said second electrode, said outer periphery of said insulating means being complementary in size and shape generally to said inner periphery of said cylindrical shell, said insulating means being in turn supported by said cylindrical shell.
 10. The combination as claimed in claim 9 wherein said last mentioned insulating means comprises glass melamine, and fusible elements, said rim and said hub comprising an integrated stamped silver based material adapted to be mounted perpendicularly and radially on said second electrode and to be easily soldered to said second electrode and said cylindrical shell, a retaining ring, said last mentioned insulating means being keyed to said cylindrical shell to prevent substantial rotary movement after assembly and retained in a generally fixed longitudinal position by said retaining ring disposed on said inner perimeter of said cylindrical shell. 